Some bacteria can gain energy by transferring electrons from a low-potential substrate such as for example, glucose, to a high-potential electron acceptor such as for example, molecular oxygen (O2) in a process commonly referred to as respiration. In eukaryotic cells, mitochondria obtain energy in the form of ATP through the processes of oxidation and phosphorylation, commonly referred to as oxidative phosphorylation. Gram-negative bacteria such as Pseudomonas aeruginosa function similarly to the eukaryotic mitochondria in producing energy. P. aeruginosa is a Gram-negative, rod-shaped bacterium with a single polar flagellum. An opportunistic human pathogen, P. aeruginosa is also an opportunistic pathogen of plants. P. aeruginosa is capable of growth at ranges of 4° C. to 42° C. It can live in diesel fuel and jet fuel where it is a hydrocarbon utilizing microorganism. It can also metabolize high nitrate-containing organic materials. P. aeruginosa derive electrons from a myriad of carbon sources and can derive electrons in aerobic, anaerobic and anaerobic fermentative processes (e.g., with arginine or pyruvate). In anaerobic growth, P. aeruginosa cells continue to couple oxidation and phosphorylation to gain energy.
In forms of microbial fuel cells, a microbe donates electrons to an anode rather than the natural recipient molecule such as oxygen, nitrate, or sulfate. Various types of microbes including bacteria and fungi have been demonstrated to generate electrical energy during metabolism, but microbial fuel cells most commonly utilize bacteria such as Geobacter or Shewanella. Geobacter cells respond to high microbial density in such a way as to interfere with large surface area biofilm formation.
In forms of microbial fuel cells, metabolic processes in the microbe generate energy in the form of electrons, especially in the anaerobic biofilm mode of growth. Rather than utilizing the energy, in a microbial fuel cell the microbe donates the electrons from a myriad of metabolized substrates to the anode for transfer through an electrical circuit. The electrical circuit carries electricity through a load, which represents work to be performed by the electron flow. The load may be a light emitting device, machinery, LCD, electrical appliance, battery charger, and many other devices.
Generally, microbes such as bacteria utilize a coenzyme known as nicotinamide adenine dinucleotide or NAD+ to accept electrons from, and thus oxidize, a feedstock or substrate. The NAD+ cleaves two hydrogen atoms from a reactant substrate. The NAD+ accepts one of the hydrogen atoms to become NADH and gains an electron in the process. A hydride ion, or cation, is released. The equation is as shown below, where RH2 is oxidized, thereby reducing NAD+ to NADH. RH2 could represent an organic substrate such as glucose or other organic matter such as organic waste.RH2+NAD+→NADH+H++R  Eq. 1NADH is a strong reducing agent that the bacteria use to donate electrons when reducing another substrate. NADH reduces the other substrate and is concurrently reoxidized into NAD+. In the natural state, the other substance may be oxygen or sulphate. In a microbial fuel cell the other substance may be a mediator or an anode. A mediator transfers electrons to the anode. The electrons, prevented from moving directly from the anode to the cathode, transfer to the cathode through an external electrical circuit and through the load perform useful work.